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Commit e2e32241 authored by Jesper Dangaard Brouer's avatar Jesper Dangaard Brouer Committed by Daniel Borkmann
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samples/bpf: xdp2skb_meta comment explain why pkt-data pointers are invalidated 



Improve the 'unknown reason' comment, with an actual explaination of why
the ctx pkt-data pointers need to be loaded after the helper function
bpf_xdp_adjust_meta().  Based on the explaination Daniel gave.

Fixes: 36e04a2d ("samples/bpf: xdp2skb_meta shows transferring info from XDP to SKB")
Reported-by: default avatarDaniel Borkmann <daniel@iogearbox.net>
Signed-off-by: default avatarJesper Dangaard Brouer <brouer@redhat.com>
Signed-off-by: default avatarDaniel Borkmann <daniel@iogearbox.net>
parent cda18e97

samples/bpf/xdp2skb_meta_kern.c

+5 −3
Original line number Original line Diff line number Diff line
@@ -35,15 +35,17 @@ int _xdp_mark(struct xdp_md *ctx) 
	void *data, *data_end;

	void *data, *data_end;

	int ret;

	int ret;





	/* Reserve space in-front data pointer for our meta info.

	/* Reserve space in-front of data pointer for our meta info.

	 * (Notice drivers not supporting data_meta will fail here!)

	 * (Notice drivers not supporting data_meta will fail here!)

	 */

	 */

	ret = bpf_xdp_adjust_meta(ctx, -(int)sizeof(*meta));

	ret = bpf_xdp_adjust_meta(ctx, -(int)sizeof(*meta));

	if (ret < 0)

	if (ret < 0)

		return XDP_ABORTED;

		return XDP_ABORTED;





	/* For some unknown reason, these ctx pointers must be read

	/* Notice: Kernel-side verifier requires that loading of

	 * after bpf_xdp_adjust_meta, else verifier will reject prog.

	 * ctx->data MUST happen _after_ helper bpf_xdp_adjust_meta(),

	 * as pkt-data pointers are invalidated.  Helpers that require

	 * this are determined/marked by bpf_helper_changes_pkt_data()

	 */

	 */

	data = (void *)(unsigned long)ctx->data;

	data = (void *)(unsigned long)ctx->data;